This invention relates generally to a system for measuring the resistivity of subsurface earth formations. More specifically, the invention relates to methods and apparatus for measuring with an increased depth of investigation, the resistivity in relatively thin formation beds or zones.
Electric well logging of subsurface earth formations is conducted by various types of instruments common in the art. One such instrument is a focused or guard electrode system. This instrument is designed to produce a measurement of formation resistivity in boreholes containing conductive drilling fluids. Examples of such instruments are illustrated in U.S. Pat. Nos. 2,347,794 and 2,712,628.
The focused electrode instrument has a cylindrical current electrode positioned between two relatively long guard electrodes. A survey current emits from the center electrode. An auxiliary current of the same polarity is applied to the guard electrodes. The current to the guard electrodes is automatically and continuously adjusted to maintain a zero voltage potential difference between the center electrode and guard electrodes, focusing the current emanating from the center, current electrode laterally into the earth formation. A drop in potential is created by the flow of survey current through the surrounding formation to a remote current return electrode. This potential difference is related to the resistivity of the formation.
This form of instrument has proven to be highly reliable in measuring formation resistivity of zones having a bed thickness greater than two feet. However, this design has provided less than satisfactory results in measuring resistivity in formation zones having a thickness less than two feet (relatively thin beds) and particularly when attempting to measure the resistivity of a thin formation zone lamenated between formation zones having a high resistivity contrast, such as a sand zone, having a relatively high resistivity, laminated between two shale zones, having a substantially lower resistivity, or vice versa. One proposed method to modify this instrument to obtain better measurements of the resistivity of these thin beds is to reduce the length of the cylindrical, center current emitting electrode from its normal length of approximately six inches. However, it can be shown that further reducing the electrode length under six inches does not materially improve the thin bed resolution due to the spreading of the measure current in the borehole before entering the earth formations. Thus, this modification does not materially enhance thin bed resolution.
This and other disadvantages are overcome by the present invention which provides methods and apparatus for measuring the resistivity in relatively thin formation beds, particularly when laminated between formation zones having resistivity contrasts.